This invention relates to transducers and more particularly to an improved construction of an acousto-optical transducer (Bragg cell).
It is known that bulk acoustic waves, primarily longitudinal waves, are generated in a Bragg cell by the application of microwave energy to the cell. (Bulk waves are comprised of longitudinal waves, defined by particle motion in the direction of wave propagation, and shear waves in which particle motion occurs transversely of the direction of propagation). In accordance with prior practice, microwave energy is applied to a multi-layer thin-film zinc oxide (ZnO) interdigital transducer deposited on the body of the cell or, alternatively, to an ion-polished piezoelectric crystal bonded to the cell body. Both approaches have major difficulties, particularly in capital investment and fabrication skill requirements.
It is also known in the prior art that a transverse electric field applied to the surface of a piezoelectric body excites bulk shear waves in the body. This is the characteristic of a prior art device having a plate-like electrode on one surface of the piezoelectric body connected to a microwave energizing source. The purpose of such devices is to measure the properties of the material by such well-known techniques as the Schaefer-Bergmann process described in an article entitled "Dielectric Materials for Device Application" by Spencer et al, Proceedings of IEEE, Vol. 55, No. 12, Dec. 1967, pages 2105-07.
Finally, surface acoustic wave (SAW) devices are well known in which surface (Rayleigh) acoustic waves are launched and received by IDT conductive arrays applied at spaced locations to the surface of an acoustic medium, such as a piezoelectric crystal. These devices typically are used as delay lines for low frequency RF signals in the 50-400 MHz range, the acoustic wave propagating along and adjacent to the medium surface from the input IDT to the output IDT.
This invention is directed to an improved microwave Bragg cell construction involving a novel combination of the foregoing principles.